My research aims to understand the tumorigenic role of fibroblast activation protein (FAP), a membrane-bound serine protease with dipeptidyl peptidase (DPP) and endopeptidase/collagenase activities. FAP is unique, in that it is expressed in cancer associated fibroblasts (CAFs) in 90% of all human epithelial derived cancers, but not in normal or transformed (malignant/neoplastic) epithelial cells. Importantly, unpublished work from our lab (J.Tchou and E. Puri) shows that FAP can be used as a robust pan-CAF marker in human breast cancer, it being superior to other CAF markers in its selectivity and specificity. CAFs play roles in ECM remodeling, angiogenesis and stromagenesis, and are important players in tumor growth and metastasis. A wide majority of reports on FAP activity describe correlations between FAP expression and poor prognosis, making FAP inhibition an appealing potential target for cancer therapeutics. Our lab recently demonstrated that FAP promotes tumorigenesis in an autochthonous mouse model of mutant K-ras- driven lung cancer and syngeneic transplant models of colon and pancreatic cancer and that the reduction in tumor growth in FAP-deficient mice was associated with enhanced accumulation of disorganized collagen, aberrant integrin signaling and reduced stromagenesis and angiogenesis. However, the mechanisms by which FAP promotes tumorigenesis are yet to be defined and its role in metastasis has not yet been investigated in any tumor type. Our preliminary data indicate that FAP mediates the proteolytic processing of the characteristic 3/4 and 1/4 fragments of collagen generated by collagenase mediated cleavage (eg. MMP-1). A central hypothesis of this proposal is that FAP promotes tumorigenesis through its protease activity by playing a critical role in ECM remodeling. Specifically, I hypothesize that by targeting collagen for degradation and turnover, FAP regulates matrix-dependent signaling of tumor cell proliferation, stromagenesis, angiogenesis and metastasis. To test these hypotheses I will compare tumorigenesis and metastasis of MMTV-Neu driven autochthonous mammary tumors in FAP wild-type mice, FAP-null mice (FAP-knock-ins expressing luciferase in place of the FAP gene) and knock-in mice expressing an enzymatic dead FAP mutant (FAPS624A) or a FAP mutant (FAPA657S) that retains DPP activity but lacks endopeptidase/collagenase activity. Therefore, by making use of these enzymatically defective FAP mice, this study will also examine the mechanism by which FAP acts on ECM remodeling. These novel FAP mouse strains have been produced in the lab and are currently breeding, while the MMTV-Neu mouse is commercially available. ! PUBLIC HEALTH RELEVANCE: Breast cancer is the second leading cause of cancer deaths in women today. The costs of cancer are a financial and mental toll on people diagnosed with cancer, their families, and society as a whole. In 2005 the burden of breast cancer in the U.S. was $12.096 billion in lost time and economic productivity (NCI, 2009/2010), therefore it is imperative to identify new molecular targets for early prognosis, treatment and eradication of this devastating disease. This research aims to better understand the causes and mechanisms underlying tumor initiation, progression and metastasis by studying the relationship between mammary tumor cells and their microenvironment. The research proposed will define the protumorigenic activity of a tumor stromal cell protease and extracellular matrix remodeling in an autochthonous breast cancer model. Results of the proposed studies will reveal novel mechanisms through which this protease regulates tumor development and therefore its value as a target for cancer therapeutics. Further, our studies may lead to an early breast cancer diagnostic biomarker that could be used in the clinic to save time, resources and most importantly, life. !